What motherboard do you use? I assume it's a dual socket. I really wish mine had more than 4 DIMM slots :-/

What kind of programs or more importantly, how many of them are running at the same time that you can justify needing that much RAM?

I use Windows as my host OS but run linux in a VM. I develop on both platforms. The main reason I need the RAM is because I work with some very large databases and I want most of the data loaded in memory rather than read from/written to my SSD

I have 2x4GB Crucial Ballistix Sport DDR3 1600Mhz, picked it up dirt cheap and never looked back. My motherboard can support up to 32GB but I've never noticed any lag with what I have so in my case it's all i need... for now